Engine Lubrication and Cooling During Hybrid Vehicle Operation

Rising fuel prices and changes to CO₂ and fuel economy legislation have prompted an interest in the electrification of vehicles since this can significantly improve vehicle tailpipe CO₂ emissions over homologation test cycles. To this end plug-in hybrid electric vehicles (PHEVs) and range extended electric vehicles (REEVs) have been introduced to the market. The operation of the engines in these vehicles differs from conventional vehicles in several key ways. This study was conducted to better understand how the engine design and control strategy of these vehicles affects the temperature and operating regimes experienced by engine crankcase lubricants.

A Toyota Prius Plug-in PHEV and GM Volt REEV were tested on a chassis dynamometer over several legislated and pseudo ‘real world’ drive cycles to determine the operating strategy and behaviour of the powertrain. The lubricant and coolant temperatures were monitored, together with other key control parameters. Tests were completed with both hot and cold engine starts at 25°C and −7°C test cell temperatures in charge-depleting and charge-sustaining operating modes.

The key findings for both vehicles were:

• The vehicles operate primarily as battery electric vehicles (BEVs) until their range is exhausted - they then switch to HEV operation where the internal combustion engine and the electric traction motor, together or independently provide the propulsive effort.
• The engines stop and start regularly when operating at low load conditions in charge-sustaining mode.
• The engines do not generally run below 1250 rev/min or above 3500 rev/min.
• The lubricant temperature can be significantly lower than a conventional vehicle but is strongly dependant on drive cycle and vehicle design.
• A complex warm-up strategy protects the engine from high load and speed operation from cold while heating the after treatment devices efficiently.